Interest in fuel cell batteries as power sources for portable electronic devices has grown. A fuel cell is an electrochemical cell that uses materials from outside the cell as the active materials for the positive and negative electrode. Because a fuel cell does not have to contain all of the active materials used to generate electricity, the fuel cell can be made with a small volume relative to the amount of electrical energy produced compared to other types of batteries.
Fuel cells can be categorized according to the type of electrolyte used, typically one of five types: proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric-acid fuel cell (PAFC), solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC). Each of these types of fuel cell can use hydrogen and oxygen as the active materials of the fuel cell negative electrode (anode) and positive electrode (cathode), respectively. Hydrogen is oxidized at the negative electrode, and oxygen is reduced at the positive electrode. Ions pass through an electrically nonconductive, ion permeable separator and electrons pass through an external circuit to provide an electric current.
In some types of hydrogen fuel cells, hydrogen is formed from a hydrogen-containing fuel supplied to the negative electrode side of the fuel cell. In other types of hydrogen fuel cells, hydrogen gas is supplied to the fuel cell from a source outside the fuel cell.
A fuel cell system can include a fuel cell battery, including one or more fuel cells (such as in a fuel cell stack), and a gas source, such as a gas tank or a gas generator. Gas generators that supply gas to a fuel cell can be an integral part of a fuel cell system, they can be removably coupled to the fuel cell system, or they can include replaceable components containing reactants. A removable gas generator can be replaced with another one when the gas producing reactants have been consumed. Removable gas generators can be disposable (intended for only a one-time use) or refillable (intended for use multiple times) to replace consumed reactant materials.
Hydrogen generators can produce hydrogen using a variety of hydrogen containing materials and a variety of methods for initiating the release of hydrogen therefrom. Hydrogen gas can be evolved when a hydrogen containing material reacts. Examples of hydrogen containing materials include liquid or gaseous hydrocarbons (such as methanol), hydrides (such as metal hydrides and chemical hydrides), alkali metal silicides, metal/silica gels, water, alcohols, dilute acids and organic fuels (such as N-ethylcarbazone and perhydrofluorene). A hydrogen containing compound can react with another reactant to produce hydrogen gas, when the reactants are mixed together, in the presence of a catalyst, heat or an acid, or a combination thereof.
In selecting reactants for use in a hydrogen generator, consideration may be given to the following: (a) stability during long periods of time when the hydrogen generator is not in use, (b) ease of initiation of a hydrogen generating reaction, (c) the amount of energy that must be provided to sustain the hydrogen generating reaction, (d) the maximum operating temperature of the hydrogen generating reaction, and (e) the total volume of hydrogen that can be produced per unit of volume and per unit of mass of the reactant(s).
Some hydrogen containing compounds can be heated to evolve hydrogen in a chemical decomposition reaction. A hydrogen generator using such types of reactants can be advantageous with regard to the volume of hydrogen that can be produced compared to other types of hydrogen generators such as those with a liquid reactant.
An object of the present invention is to provide a hydrogen generator with one or more of the following features: inexpensive and easy to manufacture, safe to store and use, able to produce a large total volume of hydrogen gas per unit of mass and per unit of volume of the hydrogen generator, able to control the supply of hydrogen on an as needed basis, able to operate at or below a desired maximum temperature, at least a portion of the hydrogen generator in a fuel cell system can be replaced after hydrogen containing materials have been consumed, and durable and reliable for a long period of time.
In one aspect of the invention, there is provided a hydrogen generator, the hydrogen generator including a cartridge, a compartment configured to removably contain the cartridge, and an initiation system. The cartridge includes a casing; a plurality of pellets, each comprising at least one material capable of releasing hydrogen gas when heated; a plurality of solid heat transfer members, each in direct contact with but not penetrating the casing and capable of conducting heat from the casing to the at least one hydrogen containing material; a hydrogen outlet in the casing; and a hydrogen flow path from each fuel pellet to the hydrogen outlet. The compartment includes a housing with a wall; a hydrogen outlet through the housing; a cavity within the housing within which the cartridge can be disposed; and a plurality of heating elements disposed within the housing, such that when the cartridge is disposed within the cavity each heating element is in contact with an outer surface of the cartridge casing and disposed so that heat can be conducted from the heating element, through the casing and to a heat transfer member, which can conduct the heat to a portion of the at least one hydrogen containing material not in contact with the casing. The initiation system includes the heat transfer members, the heating elements, and circuitry for conducting an electric current to the heating elements, such that the electrical current can be applied selectively to one or more heating elements for generating heat to selectively heat one or more pellets to initiate a release of hydrogen gas. Embodiments can include one or more of the following features:                the cartridge has a cylindrical shape;        the cartridge has a prismatic shape;        the cartridge and the compartment cooperate so the cartridge can be inserted into the compartment only such that the heating elements and the heat transfer members are aligned for conducting heat from the heating elements, through the casing to corresponding heat transfer members;        the plurality of pellets is disposed in a plurality of layers; each layer can include a plurality of pellets;        the plurality of pellets is disposed in a single layer;        at least a portion of each heat transfer member is disposed on a pellet surface;        the heat transfer members are partially disposed within the pellets;        each heat transfer member has a cartridge casing contact portion;        each heat transfer member includes aluminum;        each heat transfer member includes a layer of pyrolitic carbon in contact with a pellet;        the heat transfer members are in pressure contact with an inside surface of the casing;        the pellets are disposed in one or more layers and a thermally insulating material with a thermal conductivity of less than 10 watts/(meter·Kelvin) is disposed between adjacent pellets in a layer of pellets; a layer of the thermally insulating material can separate adjacent layers of pellets; pellet surfaces can be coated with a layer of the thermally insulating material;        the portion of the cartridge casing that makes contact with the heating element comprises stainless steel or aluminum;        the housing includes a material with an electrical conductivity at 293° K of less than 10-10 ohm-1·meter 1 and a thermal conductivity of less than 10 watts/(meter·Kelvin);        the heating elements are disposed on an inside surface of at least one of the wall and the lid the housing;        the heating elements make pressure contact with the outer surface of the cartridge casing when the cartridge is disposed within the compartment;        the cartridge includes means for maintaining contact between the heat transfer members and the pellets as the hydrogen generator is being used;        the cartridge includes means for maintaining a desired alignment between the heat transfer members and the heating elements as the hydrogen generator is being used;        the hydrogen flow path includes a channel extending through all layers of pellets; the hydrogen flow path can include a central channel; the hydrogen flow path can comprise more than one channel;        at least one filter is disposed in the hydrogen flow path;        the cartridge is enclosed in the casing prior to the cartridge being disposed in the cavity; the cartridge can be sealed in the casing prior to the cartridge being disposed in the cavity; the cartridge can include a foil seal over the hydrogen outlet valve prior to insertion of the cartridge into the compartment; the foil seal can be broken upon insertion of the cartridge into the compartment;        the at least one hydrogen containing material is selected from the group consisting of a material that can absorb and desorb hydrogen and a material that can react to produce hydrogen gas upon thermal decomposition;        the pellets further include an ignition material, preferably at least one material capable of reacting exothermically selected from the group consisting of metal/metal oxide multilayers, a metal/metal multilayered thin film, an autoignition composition, a gel of a metal and water, and a gel of metal and water in combination with sodium borohydride; and        the pellets do not contain a catalyst for the release of hydrogen gas.        
In another aspect of the invention, there is provided a fuel cell system including a fuel cell battery and the hydrogen generator as described above. Embodiments can include one or more of the following features:                a portion of the initiation system is outside the hydrogen generator; and        the initiation system is configured to monitor at least one of temperature and pressure and selectively heat one or more pellets to provide hydrogen gas as needed by the fuel cell battery.        
Unless otherwise specified, the following definitions and methods are used herein:                a solid heat transfer member is heat transfer member that is not hollow and cannot be used to transport a fluid heating medium therethrough;        a thermally insulating material is a poor conductor of heat; a poor conductor of heat has a thermal conductivity of less than 10 watts/(meter·Kelvin), preferably less than 1 watt/(meter·Kelvin);        a poor electrical conductor has an electrical conductivity at 293° K of less than 10-10 ohm-′·meter-1; and        penetrate means to pass through (e.g., between inner and outer surfaces or from one side to another side such as through a seam or joint).        
Unless otherwise specified herein, all disclosed characteristics and ranges are as determined at room temperature (20-25° C.).